Malaria remains a major public health problem in the world with nearly half the world's population living in malaria endemic areas. It is estimated that there are some 500 million new infections every year with 2-3 million deaths, especially of young children, annually.
The development of malaria vaccines remains a highly desirable goal as Plasmodium falciparum, which is responsible for malaria and its transmission, continues to become resistant to new generation anti-malaria drugs. Extensive research efforts are underway to find a vaccine which is effective but so far these efforts have been to no avail. Various targets for current vaccine efforts include the infective sporozoites, erythrocytic asexual stages and the sexual stages which are responsible for transmission of malaria. Some of the stumbling blocks in this as-yet unattained goal are antigenic variation and polymorphism of both T and B cell epitopes, and the predominantly conformational nature of the epitopes in the target antigens (1,2). Most antigens identified thus far as the targets for immunity directed against the parasite stages found in the mosquito vector are in fact conformational, and this makes the development of an effective vaccine particularly difficult. This form of immunity, which is also known as transmission-blocking immunity (TBI), is largely antibody mediated and operates in the mosquito midgut to block either fertilization of female gametes by the male gametes or results in the interruption of subsequent development of zygotes (3). Target antigens of TBI have been defined using monoclonal antibodies (MAbs). They include a set of three gamete surface proteins of apparent Mr 230, 48 and 45 kDa identified by fertilization blocking antibodies (4-6), and a single protein of 25 kDa present in the surface of zygotes/ookinetes, recognized by post-fertilization blocking antibodies (7-9). The gamete surface antigens are produced by the blood-stage gametocytes and the 25 kDa protein predominantly after initiation of gametogenesis and fertilization in the mosquito midgut (10-12).
In almost every case, the epitopes recognized by transmission blocking MAbs have been found to be reduction-sensitive, hence conformational in nature (3). It is believed that the conformational nature of the epitopes recognized by transmission blocking antibodies largely accounts for the fact that such antibodies have not proven useful in cloning the genes from prokaryotic expression libraries. Nevertheless, even if genes are cloned using other approaches e.g., by means of oligonucleotides based on protein microsequence data, reproducing such conformational epitopes in the recombinant expression systems would be difficult, if not impossible.
On the other hand, the identification of reduction-insensitive continuous epitopes of blocking antibodies would greatly facilitate development of a recombinant or peptide-based subunit vaccine. The relevant peptide epitopes can be chemically defined and synthesized or expressed, to develop a subunit vaccine. The vaccine can also be designed by the addition of appropriate helper T lymphocyte epitopes (which are also comprised of a linear stretch of 7-15 amino acids) or the equivalent, to stimulate the appropriate immune response.